Optical material comprising silica glass and method for producing said optical material, silica glass modification method for suppressing rayleigh scattering of silica glass, silica glass modification method for suppressing nonlinear optical effect of silica glass, method for improving refractive index of silica glass, and method for producing optical component

Abstract

This optical material comprises silica glass having no voids with a diameter of 5.0 Å or greater when analyzed by a positron annihilation lifetime measuring device.

Description

Optical material made of silica glass and method for manufacturing the same; method for modifying silica glass to suppress Rayleigh scattering of silica glass; method for modifying silica glass to suppress nonlinear optical effects of silica glass; method for improving the refractive index of silica glass; and method for manufacturing optical components.

[0001] This invention relates to silica glass in which Rayleigh scattering and / or nonlinear optical effects are suppressed. Furthermore, this invention relates to silica glass with an increased refractive index. Also, this invention relates to a method for modifying silica glass and optical components using the modified silica glass.

[0002] Silica glass, widely used as a base material for optical communication fibers, has a higher SiO2 content compared to general multi-component glasses. ２ Techniques have been established to create highly pure, homogeneous silica glass structures with low levels of structural defects and moisture. Furthermore, the thermal expansion coefficient of silica glass is 5 × 10⁻⁶. －７ K －１ The silica glass fiber has an extremely small optical gap of 6 eV or more. As a result, linear and nonlinear physical and optical changes such as thermal expansion due to light absorption and lensing effects due to changes in refractive index are less likely to occur for light across a wide wavelength range. In just about 10 years, from the time that optical communication fibers were first made from silica glass until around 1980, the transmission loss in the communication wavelength band (hereinafter simply referred to as "transmission loss of optical communication fibers") was reduced from 20 dB / km to 0.2 dB / km, and with the global transition to an information society, silica glass optical communication fibers are now spread throughout the world.

[0003] If the transmission loss of optical communication fibers could be reduced by another order of magnitude, it would not only significantly reduce the need for optical signal amplifiers, but it is also said that quantum communication, for which repeater fabrication is currently difficult, could become widespread. However, since 1980, the transmission loss of silica glass optical communication fibers has remained stagnant, with only a reduction of about 0.01 dB / km reported every two to three years. Therefore, further reduction of the transmission loss of silica glass optical communication fibers is an important research topic.

[0004] Over 80% of the transmission loss in silica glass optical communication fibers is due to Rayleigh scattering loss, with only a small amount of transmission loss attributable to factors such as structural mismatch and absorption. One method to suppress Rayleigh scattering loss is to adjust the virtual temperature (T) of the silica glass. ｆ Methods to lower ) are known (e.g., Non-Patent Document 1). ｆ To reduce the Rayleigh scattering loss, it is necessary to either freeze the glass structure into a stable state over a long period of time, or to add fluorine, hydroxyl groups, alkali ions, etc., to reduce viscosity and allow the structure to reach a stable state more quickly. However, the former requires lowering the spinning speed of the optical communication fiber, which is not practical considering the current production process of optical communication fibers that are spun at speeds close to the speed of sound. In the latter case, crystallization occurs or compositional fluctuations are amplified, which actually increases the scattering loss. For these reasons, it has been considered difficult to control the Rayleigh scattering loss of silica glass optical communication fibers to the desired low level.

[0005] K. Saito, M. Yamaguchi, H. Kakiuchida, AJ Ikushima, K. Ohsono, and Y. Kurosawa, “Limit of the Rayleigh scattering loss in silica fiber,” Appl. Phys. Lett. 83(25), 5175-5177 (2003)

[0006] Apart from the transmission loss problem mentioned above, optical communication fibers are also facing the problem of signal degradation due to nonlinear optical effects. Specifically, with the dramatic increase in the amount of information transmitted through optical communication in recent years, the amount of light passing through optical communication fibers is steadily increasing. This increase in light intensity is causing a significant problem: even in the optical communication wavelength band, the silica glass constituting the optical communication fiber exhibits a third-order nonlinear refractive index effect (a phenomenon in which the refractive index changes in proportion to the light intensity, and this third-order nonlinear refractive index effect causes phase modulation of the optical signal). It is easy to imagine that this problem will become even more apparent in the next few years, given the predicted explosive increase in communication volume. Problems caused by the third-order nonlinear refractive index effect are also known in fields other than optical communication. For example, silica glass (including doped glass) used in parts such as lenses, photomasks, reticles, and substrates in transmissive semiconductor exposure equipment, and optical fibers for fiber lasers, are considered problematic due to the action of laser light with shorter wavelengths and higher intensity than the optical communication wavelength band.

[0007] On the other hand, silica glass used as an optical material, such as in optical communication fibers, is required to have light-trapping properties, and from this perspective, a high refractive index (linear refractive index, which does not depend on light intensity) is required. However, when the refractive index is increased, the refractive index element is incorporated into the value of the nonlinear refractive index (refractive index that depends on light intensity), and in principle, the nonlinear refractive index also increases. Therefore, it is difficult to achieve both suppression of the nonlinear refractive index and improvement of the refractive index in silica glass.

[0008] In one aspect, the present invention aims to provide an optical material made of silica glass that is less prone to Rayleigh scattering loss. In connection with this, the present invention aims to provide an optical material made of silica glass that is less prone to Rayleigh scattering loss, for forming an optical component (optical part) in which Rayleigh scattering loss is further suppressed, and a method for modifying silica glass to suppress Rayleigh scattering loss of silica glass. In another aspect, the present invention aims to provide an optical material made of silica glass that is less prone to nonlinear optical effects. In connection with this, the present invention aims to provide an optical material made of silica glass that is less prone to nonlinear optical effects, for forming an optical component in which nonlinear optical effects are further suppressed, and a method for modifying silica glass to suppress nonlinear optical effects of silica glass. In yet another aspect, the present invention aims to provide a method for improving the refractive index of silica glass.

[0009] The above problems of the present invention are solved by the following means: [1] An optical material made of silica glass that does not have voids of 5.0 Å or more in diameter as measured by positron annihilation lifetime measurement. [2] The optical material according to [1], wherein the silica glass does not have ordered structure peaks of 8.0 Å or more as measured by high-energy X-ray diffraction. [3] The optical material according to [1] or [2], wherein the silica glass contains at least one atom of F, Cl, Al, P, Na, and K, and / or a hydroxyl group (OH). [4] The optical material according to any one of [1] to [3], wherein the optical material is an optical material for suppressing Rayleigh scattering loss. [5] The optical material according to any one of [1] to [3], wherein the optical material is an optical material for suppressing nonlinear optical effects. [6] The optical material according to any one of [1] to [5], wherein the optical material is an optical material for suppressing an increase in nonlinear refractive index while increasing the refractive index. [7] The optical material according to any one of [1] to [6], wherein the optical material is a material for forming optical components. [8] The optical material according to [7], wherein the optical component is an optical communication fiber. [9] The optical material according to [7], wherein the optical component is an optical component used in a transmissive semiconductor exposure apparatus.

[10] The optical material according to [9], wherein the optical component used in the transmissive semiconductor exposure apparatus is a lens, a photomask, a reticle, or a substrate.

[11] The optical material according to [7], wherein the optical component is an optical fiber for a fiber laser.

[12] Intensity 1000 W / cm ２An optical material according to any one of [1] to

[11] for injecting the above laser light.

[13] An optical material according to any one of [1] to

[12] , wherein the refractive index of the optical material at a wavelength of 1030 nm is 1.452 or more.

[14] A method for manufacturing an optical material, comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling it.

[15] A method for modifying silica glass to suppress Rayleigh scattering of silica glass, comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling it.

[16] A method for modifying silica glass to suppress nonlinear optical effects of silica glass, comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling it.

[17] A method for modifying silica glass to increase the refractive index of silica glass while suppressing an increase in the nonlinear refractive index of the silica glass, comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling the silica glass.

[18] A method for improving the refractive index of silica glass, comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling the silica glass.

[19] The method according to any one of

[14] to

[18] , wherein the silica glass obtained by the method does not have voids with a diameter of 5.0 Å or more as determined by analysis by positron annihilation lifetime measurement.

[20] The method according to any one of

[14] to

[19] , wherein the pressure is applied by introducing an inert gas.

[21] The method according to any one of

[14] to

[20] , wherein the heating and melting, the application of pressure, and the rapid cooling are performed using a hot isostatic processing apparatus.

[22] The method according to any one of

[14] to

[19] , wherein the heated and molten silica glass is the core material of an optical fiber, and the pressure is applied to the heated and molten core material by causing the cladding material surrounding the core material to shrink in volume by cooling.

[23] A method for manufacturing an optical component, comprising forming an optical component using an optical material or modified silica glass obtained by the method according to any one of

[14] to

[22] .

[24] A method for manufacturing an optical fiber, comprising obtaining an optical material or modified silica glass obtained by the method described in

[22] as the core of an optical fiber.

[0010] In this invention, when we refer to "silica glass," we mean pure silica glass (SiO ２ This means that in addition to Si and O themselves, the material also includes those containing small amounts (for example, 5.0% by mass or less, preferably 3.0% by mass or less, and even more preferably 1% by mass or less) of elements other than Si and O (for example, Ti, Zr, Al, F, OH, Cl, Na, K, P, N, etc.).

[0011] The optical material of the present invention, when used as a forming material for optical communication fibers, can sufficiently suppress Rayleigh scattering loss in the resulting optical communication fibers, and is less prone to signal degradation due to nonlinear optical effects. When used as a forming material for optical components (optical elements) in transmissive semiconductor manufacturing equipment, the optical material of the present invention can prevent defects and point absorption caused by increased light intensity due to self-focusing effects. Furthermore, while large nonlinear optical effects (third-order nonlinear refractive index effects) lead to non-uniform refractive index distribution, the optical material of the present invention can prevent such non-uniformity of refractive index, thus preventing distortion of lithographic images. As a result, repair and manufacturing downtime for replacing optical components are unnecessary, resulting in significant economic benefits. In addition, the optical material of the present invention has low Rayleigh scattering loss, allowing for sufficient exposure even with relatively low-intensity incident light, and if the incident light intensity is constant, an exposure image can be created with a shorter exposure time. Moreover, since fiber lasers handle light orders of magnitude stronger than that used in conventional optical communication, using the optical material of the present invention as a forming material for fiber laser optical fibers makes the fiber less susceptible to damage even when the light intensity is increased. The optical material of the present invention demonstrates suppression of nonlinear optical effects using laser light at wavelengths of 1030 nm and 800 nm as an example. The third-order nonlinear optical constant of the material is strongly correlated with the absorption coefficient and increases monotonically as it approaches the wavelength with a large absorption coefficient. The fact that the nonlinear optical constant is small for a certain single wavelength usually means that a similar reduction in the third-order nonlinear optical constant can be expected at other wavelengths as well. Therefore, this strongly suggests that the third-order nonlinear optical refractive index is small even for fiber lasers using various wavelengths, and it can be suitably used as a fiber for optical transmission or laser oscillation that is durable against stronger laser light at any wavelength. In addition, the optical material of the present invention has low Rayleigh scattering loss and can sufficiently increase the output light intensity even when the laser light intensity of the light source is relatively low. Furthermore, if the incident light intensity is constant, the laser output is increased even more, making it suitable as a fiber laser where high output with low energy consumption is desired. The optical material of the present invention having the above characteristics can be obtained by the manufacturing method of the optical material of the present invention.According to the method for suppressing Rayleigh scattering in silica glass of the present invention, silica glass in which Rayleigh scattering is sufficiently suppressed can be obtained. According to the method for suppressing nonlinear optical effects in silica glass of the present invention, silica glass in which nonlinear optical effects are sufficiently suppressed can be obtained. Furthermore, in one embodiment, the present invention can provide a method for improving the refractive index of silica glass.

[0012] Figure 1 is a schematic diagram illustrating the configuration of an apparatus for measuring the third-order nonlinear refractive index. Figure 2 is a graph showing an example of a signal obtained by the apparatus shown in Figure 1. Figure 3 is a graph plotting the peak height of the + signal in Figure 2 when the power of the incident laser light (1030 nm) is changed. Figure 4 is a graph showing the relationship between the third-order nonlinear refractive index γ of a silica glass sample subjected to high-temperature, high-pressure freeze treatment and the applied pressure. Figure 5 is a graph showing the relationship between the linear refractive index n of a silica glass sample subjected to high-temperature, high-pressure freeze treatment and the applied pressure. The FTIR spectrum (absorption spectrum) (upper figure) and the peaks separated by fitting, which are used to determine the virtual temperature (lower figure), are shown. Figure 6 is included for reference only, showing conventional knowledge, and does not represent the measurement results of the optical material of the present invention. Figure 7 is a graph showing the relationship between the Rayleigh scattering coefficient of a silica glass sample subjected to high-temperature, high-pressure freeze treatment and the pressure of the treatment. Figure 8 is a graph showing the change in the Raman scattering spectrum of the core glass (core material after spinning) that was subjected to pressure from the cladding during the fiber spinning process, compared to the Raman scattering spectrum of the core glass before fiber spinning (without pressure treatment). Figure 9 is a graph showing the results of measuring the total scattering spectrum of the core with fibers lined up using 61 keV high-energy X-rays at the Spring-8 BL04B2 line.

[0013] [Optical Material] The optical material of the present invention consists of silica glass that does not have voids with a diameter of 5.0 Å or more, as determined by analysis using a positron annihilation lifetime measuring device. Silica glass of such a structure has sufficiently low Rayleigh scattering loss and excellent transmittance, making it suitable, for example, as a material for forming optical communication fibers. That is, optical communication fibers obtained using the optical material of the present invention have sufficiently low transmission loss, enabling the transmission of a larger amount of information, and dramatically increasing the distance over which data can be transmitted without using an optical signal amplifier. It can also contribute to the realization of quantum communication. Furthermore, because the optical material of the present invention has the above structure, the occurrence of nonlinear optical effects (third-order nonlinear refractive index effects) can be sufficiently suppressed (i.e., the increase in nonlinear refractive index can be suppressed. In the present invention, "suppressing the increase in nonlinear refractive index" means not only suppressing the degree of increase in nonlinear refractive index, but also preventing the increase in nonlinear refractive index or reducing it). As a result, optical communication fibers obtained using the optical material of the present invention can suppress phase modulation of optical signals (degradation of optical signals) and enable highly efficient transmission of optical signals. Furthermore, due to the above-described structure of the optical material of the present invention, it is possible to effectively increase the refractive index while suppressing the increase in nonlinear refractive index, even though it does not have voids with a diameter of 5.0 Å or more (resulting in extremely small fluctuations). In other words, it is possible to achieve both the suppression of nonlinear refractive index and the improvement of refractive index, which was previously considered difficult in principle. The increase in refractive index can increase the light confinement efficiency of silica glass. Normally, when the linear refractive index n increases, the nonlinear refractive index γ also increases in principle, but this is not the case with the optical material of the present invention and the silica glass obtained by the method of the present invention. The refractive index and nonlinear refractive index of the optical material of the present invention and the silica glass obtained by the method of the present invention change in a direction suitable for handling high-intensity laser light. Also, as described above, it is preferable to form optical components used in transmission-type semiconductor exposure apparatus using the optical material of the present invention. Examples of such optical components include lenses, reticles, and substrates. Furthermore, the optical material of the present invention is also suitable as a material for forming optical fibers for fiber lasers.

[0014] A positron annihilation lifetime analyzer is a device that measures the size of voids in a sample by injecting positrons into the sample, measuring and analyzing the time it takes for the positrons and electrons to annihilate each other, and can identify extremely small void sizes. For example, DPALMS-LH (trade name, manufactured by TechnoAP) can be used as a positron annihilation lifetime analyzer. The timing of the obtained gamma ray measurement reflects the lifetime of positronium (positron-electron pair). In this invention, the positron lifetime component was decomposed into three components (τ1 to τ3) from this timing profile using the software PALSFIT. Furthermore, the lifetime of the τ3 component that affects Rayleigh scattering in silica glass was converted to the radius and diameter of the void using a model equation called the Tau-Eldrup model (Reference: H. Nakanishi and YC Jean, Positron and Positronium Chemistry, edited by DM Schrader and YC Jean (Elsvier, Amsterdam, 1988), p. 159) (for example, 1.6 ns corresponds to a radius of 0.24553 nm). In this invention, the statement that silica glass "does not have voids with a diameter of 5.0 Å or more in analysis by positron annihilation lifetime measurement" means that 30 measurement samples (for example, 10.0 mm long × 10.0 mm wide × 1.0 mm thick) are randomly cut from the silica glass, and all 30 measurement samples are analyzed by positron annihilation lifetime measurement, and voids with a diameter of 5.0 Å or more are not detected in 27 or more (27 to 30) measurement samples. In 30 measurement samples, it is preferable that no voids with a diameter of 5.0 Å or larger are detected in 28 or more (27 to 30) of the measurement samples as a result of the above analysis, more preferably that no voids with a diameter of 5.0 Å or larger are detected in 29 or more of the measurement samples, and even more preferably that no voids with a diameter of 5.0 Å or larger are detected in all 30 measurement samples.As described above, since the silica glass constituting the optical material of the present invention "has no voids with a diameter of 5.0 Å or more in analysis by positron annihilation lifetime spectroscopy", the extremely fine voids in the structure of the silica glass are reduced beyond a level at which they are merely reduced, dramatically to a level that is effectively zero.

[0015] The optical material (silica glass) of the present invention preferably has no ordered structure peak of 8.0 Å or more in analysis by high-energy X-ray diffraction (XRD). The phrase "has no ordered structure peak of 8.0 Å or more" means that when the peak intensity of FSDP (First sharp diffraction peak) corresponding to an ordered structure of 4.0 to 5.0 Å is defined as 2.00, the peak intensity of the ordered structure peak of 8.0 Å or more is 0.04 or less. Since the ordered structure peak of 8.0 Å or more is located in the tail region of FSDP, the "ordered structure peak of 8.0 Å or more" is separated by peak fitting, and when the peak intensity of the separated peak is 0.04 or less relative to the FSDP peak intensity of 2.00, it is determined that "there is no ordered structure peak of 8.0 Å or more". Here, in the present invention, the "analysis by high-energy XRD" is performed based on the obtained scattering profile by using silica glass powder as a sample, irradiating the sample with 61 keV X-rays using the BL04B2 beamline of the large synchrotron radiation facility SPring-8 (owned by RIKEN), and performing angular resolution (2θ) on the scattered X-rays. The scattering angle (2θ) is converted to wavenumber in consideration of the energy of incident X-rays, and the wavenumber q = 1.5 Å －１ Peak intensity (I) of FSDP appearing in the vicinity and 0.8 Å －１The peak intensity (II) of the peak separated by fitting the "ordered structure peak of approximately 8.0 Å" that appears nearby with a Gaussian function or Lorentz function is determined, and if the intensity ratio ((II) / (I)) is 0.04 / 2.00 or less, it is determined that the silica glass "does not have an ordered structure peak of 8.0 Å or more". In this invention, when the above high-energy X-ray diffraction (XRD) analysis is performed on 30 silica glass powder samples, it means that in 27 or more (27 to 30) silica glass powder samples, the peak intensity ratio ((II) / (I)) is 0.04 / 2.00 or less. In 30 silica glass powder samples, the above analysis preferably shows that in 28 or more measured samples, the peak intensity ratio ((II) / (I)) is 0.04 / 2.00 or less; in 29 or more measured samples, the peak intensity ratio ((II) / (I)) is 0.04 / 2.00 or less; and in all 30 measured samples, the peak intensity ratio ((II) / (I)) is 0.04 / 2.00 or less.

[0016] Furthermore, the optical material (silica glass) of the present invention may contain at least one atom of F, Cl, Al, P, Na, and K, and / or a hydroxyl group (OH). The total content of these is preferably 0 to 2000 ppm, and more preferably 0 to 1000 ppm. ppm is based on mass. By including trace amounts of F, Cl, Al, P, Na, K, and OH in this way, the heterogeneity of the silica glass network structure can be more efficiently suppressed, and the voids can be made smaller. In addition, in Fourier transform infrared spectroscopy (FTIR) analysis (using Shimadzu Corporation's IRAffinity-1S infrared spectrophotometer and AIM-8800 infrared microscope), the optical material of the present invention showed a peak in the absorption spectrum measured by transmitted light at 2267 cm⁻¹. －１ Preferably, it is 2260 cm. －１ The following is more preferable: 2258 cm －１ The following is even more preferable: 2257 cm －１ The following is even more preferable: 2256 cm －１The following are particularly preferable.

[0017] The optical material (silica glass) of the present invention preferably has a virtual temperature of 1200°C or higher, more preferably 1200 to 2400°C, and may also be 1300 to 2300°C, 1400 to 2200°C, 1500 to 2100°C, or 1600 to 2000°C.

[0018] As described above, the optical material of the present invention exhibits unique optical properties that were previously unattainable, such as effectively increasing the refractive index despite having no voids with a diameter of 5.0 Å or more (extremely small fluctuations), and suppressing the increase in nonlinear refractive index while maintaining a high refractive index. Therefore, as an optical material for applications such as optical communication fibers, it is possible to sufficiently confine light while also sufficiently suppressing phase modulation of optical signals. Consequently, it is particularly suitable for applications handling high-intensity laser light. The laser intensity of the high-intensity laser light is 1000 W / cm². ２ The above is preferable, with a load capacity of 2500 W / cm². ２ The above is even more preferable, with a load of 5000 W / cm². ２ The above is even more preferable. A preferred range for the laser intensity of the high-intensity laser light is 1,000 to 100,000 W / cm². ２ Preferably, 2500 to 50000 W / cm² ２ More preferably, 5000 to 30000 W / cm² ２ That is even more preferable.

[0019] The optical material of the present invention preferably has a refractive index of 1.452 or higher at a wavelength of 1030 nm, more preferably 1.453 or higher, also preferably 1.454 or higher, also preferably 1.455 or higher, also preferably 1.456 or higher, also preferably 1.457 or higher, also preferably 1.458 or higher, also preferably 1.459 or higher, and also preferably 1.460 or higher. This refractive index is usually 1.600 or lower. Furthermore, the optical material of the present invention preferably has a refractive index of 1.455 or higher at a wavelength of 800 nm, more preferably 1.456 or higher, also preferably 1.457 or higher, also preferably 1.458 or higher, also preferably 1.459 or higher, also preferably 1.460 or higher, also preferably 1.461 or higher, also preferably 1.462 or higher, and also preferably 1.463 or higher. This refractive index is usually 1.600 or lower.

[0020] [Method for Manufacturing Optical Materials] The method for manufacturing optical materials of the present invention employs characteristic manufacturing conditions in order to obtain a silica glass structure that does not have voids with a diameter of 5.0 Å or more, as determined by analysis using the above-mentioned positron annihilation lifetime spectroscopy device. Specifically, very high pressure is applied to heated and molten silica glass, and then it is subjected to a rapid cooling process to freeze it into a high-pressure structural state without relaxing the glass structure. For example, in a heated and molten state at 1600 to 2400°C, more preferably 1700 to 2400°C, and even more preferably 1800 to 2200°C, it can be placed under a pressure of, for example, 100 MPa or more, preferably 100 to 1500 MPa, more preferably 150 to 1200 MPa, and even more preferably 200 to 1000 MPa, and then rapidly cooled at a high cooling rate, for example, 10°C / min or more, preferably 10 to 50,000,000°C / min, more preferably 30 to 40,000,000°C / min, even more preferably 60 to 20,000,000°C / min, even more preferably 120 to 10,000,000°C / min, even more preferably 240 to 5,000,000°C / min, even more preferably 500 to 1,000,000°C / min, and even more preferably 1,000 to 500,000°C / min. This makes it possible to obtain glass that maintains its structure under high pressure. The time for applying the above pressure is not particularly limited and can be, for example, 0.0001 to 300 minutes, and preferably 0.01 to 100 minutes. In general, cells made of tungsten carbide or boron nitride are used to apply such very high pressure. However, when these cells are applied to the manufacture of the optical material of the present invention, crystal nuclei are formed at the contact surface with the silica glass, and most of the glass changes into crystals such as quartz or cristobalite. For this reason, an inert gas is preferred as the medium for applying pressure, and it is more preferable to use a noble gas such as Ar, Kr, or Xe. If He or Ne is used as a noble gas, the gas penetrates into the interior of the silica glass, making it impossible to apply pressure, and these gases remain in the voids, making it difficult to effectively contract the voids. Considering availability, Ar is preferred.Furthermore, if gases other than noble gases are used, the reaction between silica glass and the gas will induce crystal nuclei. By adopting the characteristic manufacturing conditions described above, it becomes possible to more reliably obtain silica glass structures that do not have voids with a diameter of 5.0 Å or more as defined in this invention. The above heating and melting, the application of pressure, and the rapid cooling can be carried out using a hot isostatic pressing (HIP) apparatus. An example of a hot isostatic pressing apparatus is an ultra-high pressure hot isostatic pressing (ultra-high pressure HIP) apparatus (owned by the Ultra-High Temperature Materials Research Center Co., Ltd.). This apparatus is a pressurizing apparatus that applies pressure by utilizing the synergistic effect of pressure and temperature, and since it applies isostatic pressure to the workpiece using gas pressure, the pressure acts uniformly on the workpiece, and the shape after pressurization does not change significantly from the initial shape of the workpiece, allowing it to shrink similarly.

[0021] On the other hand, by using glass with a coefficient of thermal expansion greater than that of silica glass, compressive stress can be applied to the internal silica glass by the pressure generated during contraction. As a glass with a large coefficient of thermal expansion, for example, normal glass (e.g., aluminosilicate or borosilicate glass) with a high glass transition temperature and volume contraction as it cools can be used. By this method, the optical material of the present invention can be obtained as the core of an optical fiber. Therefore, in one embodiment, the present invention provides a method for manufacturing the following optical material.

[0022] A method for manufacturing an optical material, comprising: the heated and molten silica glass being the core material of an optical fiber; applying pressure to the heated and molten core material by causing the cladding material surrounding the core material to shrink in volume by cooling; and then rapidly cooling.

[0023] [Method for modifying silica glass] An embodiment of the method for modifying silica glass of the present invention is a method for suppressing Rayleigh scattering of silica glass and also suppressing nonlinear optical effects. More specifically, there is provided a method for modifying silica glass for suppressing Rayleigh scattering or suppressing nonlinear optical effects of silica glass, the method comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling the same as described above. In another embodiment of the method for modifying silica glass of the present invention, there is provided a method for modifying silica glass for suppressing an increase in nonlinear refractive index of silica glass while increasing the refractive index of silica glass, the method comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling the same as described above. In the method for modifying silica glass of the present invention, the description of the method for producing an optical material of the present invention is directly applied to the heating and melting temperature, pressure, quenching rate (temperature decrease rate), and other preferable conditions. The optical material of the present invention can be obtained by the method for modifying silica glass of the present invention, and the obtained optical material can be used as an optical material with suppressed Rayleigh scattering, an optical material with suppressed expression of nonlinear optical effects, or an optical material with an increased refractive index, as a material for forming various optical members. In one aspect, the present invention also provides the following method for modifying silica glass.

[0024] The method for modifying silica glass (core material), wherein the heat-melted silica glass is a core material of an optical fiber, and the method comprises applying the pressure to the core material in the heat-melted state by causing a clad material around the core material to undergo volume shrinkage through temperature decrease, and then quenching the core material.

[0025] [Method for increasing refractive index of silica glass] In another embodiment of the present invention, there is provided a method for increasing the refractive index of silica glass, which comprises applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling the same as described above. By increasing the refractive index of silica glass, light can be sufficiently confined in an optical fiber using the silica glass. At the same time, if the refractive index of silica glass is increased by this method, an increase in nonlinear refractive index, which normally correlates with the refractive index, can be suppressed, so phase modulation of optical signals can also be effectively suppressed. In the method for increasing the refractive index of silica glass of the present invention, the description of the method for producing an optical material of the present invention is applied as is to the heating and melting temperature, pressure, quenching rate (temperature decrease rate), and other preferable conditions. In one aspect, the present invention also provides the following method for increasing the refractive index of silica glass.

[0026] A method for increasing the refractive index of silica glass (core material), comprising: the heat-melted silica glass is a core material of an optical fiber; the pressure is applied to the core material in the heat-melted state by causing a cladding material around the core material to shrink in volume through temperature decrease; and then the core material is quenched.

[0027] [Method for producing optical member] The method for producing an optical member of the present invention comprises forming an optical member using an optical material or modified silica glass obtained by the above-described method for producing an optical material, method for modifying silica glass, or method for increasing the refractive index of silica glass of the present invention. Examples of the optical member include, as described above, optical communication fibers, optical members for transmission-type semiconductor manufacturing apparatuses, and optical fibers for fiber lasers.

[0028] In one embodiment, the present invention provides a method for manufacturing an optical fiber, which includes obtaining an optical material or modified silica glass as the core of an optical fiber by the methods described above. For example, when forming an optical communication fiber using the optical material of the present invention, a silica glass material (ordinary) is used as the core, and silica glass containing a dopant is used as the cladding, so that in the spinning process the cladding solidifies before the core structure solidifies, and a desired high pressure is applied to the core as the cladding shrinks. In this case, by adding a viscosity-reducing dopant such as F (fluorine) or alkali metal ions to the core, a desired, substantially isotropic high pressure can be applied to the core for a sufficiently long period of time before the core solidifies, and a desired glass network structure can be created in the core. Furthermore, the spinning process may include a step in which an X-ray or ultraviolet light generator is placed in the spinning process and X-ray or ultraviolet light is irradiated onto the core glass during spinning to promote structural relaxation. As the cladding material, glass with a large coefficient of thermal expansion in a supercooled liquid state is preferred. The core glass continues to shrink in a liquid or supercooled liquid state for a while even after the cladding solidifies during the spinning process, and during this shrinkage, a desired high pressure is applied to the core from the cladding. Since the cladding glass is on the outside of the fiber, it cools before the core, but the spinning process can be designed so that the core glass reaches a temperature of 1200°C or higher when the cladding solidifies, and a pressure of about 100 to 1500 MPa can be applied during the spinning process by utilizing the shrinkage behavior of the cladding. For this reason, it is preferable that there is a large difference in thermal shrinkage between the core and the cladding. By employing such methods, it is possible to process the material into fibers and obtain an optical communication fiber with the optical material or modified silica glass of the present invention as the core. Furthermore, when forming an optical fiber for a fiber laser using the optical material of the present invention, the material can be processed into fibers in the same way as when forming the optical communication fiber described above, by applying stress (pressure) from the cladding to the core before the core structure solidifies during the fiber spinning process, thereby obtaining an optical fiber for a fiber laser.

[0029] Furthermore, when forming optical components for use in a transmissive semiconductor exposure apparatus using the optical material of the present invention, lenses can be formed by cutting the optical material of the present invention into the desired shape or curved surface and then grinding and polishing it, or photomasks, rectils, or substrates can be formed by slicing the optical material of the present invention and then polishing it.

[0030] Figure 1 is a schematic diagram illustrating the configuration of an apparatus for measuring the third-order nonlinear refractive index. In this apparatus, the change in the intensity density of incident light corresponding to the position of a silica glass sample is detected by moving the sample in the Z-axis direction. For example, when Z < 0 (Z is negative (-)), the beam diameter at the aperture position increases, and the detected light intensity detected by the detector decreases. Conversely, when Z > 0 (Z is positive (+)), the beam diameter at the aperture position decreases, and the detected light intensity detected by the detector increases. The larger the magnitude of the third-order nonlinear refractive index of the sample, the weaker the detected light intensity when Z < 0, and the stronger the detected light intensity when Z > 0.

[0031] Figure 2 shows an example of a signal obtained by measuring the third-order nonlinear refractive index using the apparatus shown in Figure 1. When a silica glass sample is moved left and right (-+) in the Z direction in the apparatus of Figure 1, a downward signal (-) peak appears on the negative Z side and an upward signal (+) peak appears on the positive Z side, as shown in Figure 2, due to the nonlinear optical effect. When the third-order nonlinear refractive index is large, the drop in the - signal peak becomes larger and the height of the + signal peak increases. Note that in this measurement, the absolute value of the change in the nonlinear optical effect cannot be measured, so the ratio with the nonlinear refractive index of the standard sample is calculated as follows. First, the detection signal of the sample (measured value) is divided by the detection signal of the standard sample (reference value). This cancels out accidental changes in light intensity such as laser fluctuations. This gives the profile of Figure 2, and the magnitude of the + and - signals from the zero point of this waveform (in Figure 2, the center value is not 0 but 0.885, which is the zero point) is read. In principle, the absolute values ​​of the + and - signals should be the same. This positive or negative signal value is measured by changing the power of the incident laser light.

[0032] Figure 3 is a graph plotting the peak height of the positive signal in Figure 2 when the power of the incident laser light (1030 nm) is varied. This slope is proportional to the third-order nonlinear refractive index. By determining this slope for each sample and comparing it to a standard sample, the third-order nonlinear refractive index can be determined. A similar graph can be obtained by plotting the magnitude of the drop in the negative signal in Figure 2.

[0033] Figure 4 is a graph showing the relationship between the tertiary nonlinear refractive index γ and the applied pressure of silica glass that has undergone high-temperature, high-pressure freezing treatment using a hot isostatic processing apparatus (treatment at 1800°C with 200-980 MPa followed by rapid cooling at 10°C / min, and pressure application time of 60 minutes). The left axis represents the tertiary nonlinear refractive index γ (Y) of silica glass with a high virtual temperature (sample with a virtual temperature of 1069°C) obtained by fabrication at atmospheric pressure of 0.1 MPa. Ａ The ratio of the third-order nonlinear refractive index γ(Y) of each silica glass subjected to high-temperature, high-pressure freezing treatment (Y / Y) Ａ ) The right axis also shows the absolute value (2.5 × 10⁻¹⁹) of the third-order nonlinear refractive index γ of the sample at a virtual temperature of 1069°C. －１６ cm ２ The nonlinear refractive index of each silica glass subjected to high-temperature, high-pressure freezing treatment using (W) is shown. As the pressure increased, the tertiary nonlinear refractive index n2 at 800 nm showed a gradual downward trend. Similarly, the tertiary nonlinear refractive index γ at 1030 nm also showed a gradual downward trend with increasing pressure. These results indicate that the nonlinear refractive index does not increase with the application of high pressure.

[0034] Figure 5 is a graph showing the relationship between the linear refractive index n and the applied pressure of silica glass subjected to high-temperature, high-pressure freezing treatment using a hot isostatic processing apparatus (treatment at 1800°C with 200-980 MPa, followed by rapid cooling at 10°C / min, and pressure application time of 60 minutes). Measurements were performed using a KPR-2000 on glass with a 5 mm × 5 mm × 5 mm right-angle surface that had been polished. As the pressure increased, the refractive index n at 800 nm and 1030 nm increased monotonically. Combining the results of Figure 5 and Figure 4, it can be seen that the silica glass obtained by the above high-temperature, high-pressure treatment and rapid cooling shows an improvement in refractive index depending on the applied pressure, and that the nonlinear refractive index tends to decrease. In other words, it has been confirmed that silica glass can achieve the unique optical property of simultaneously suppressing the nonlinear refractive index and improving the refractive index.

[0035] The silica glass samples shown in Figures 4 and 5, which underwent high-temperature, high-pressure freeze treatment (treated at 1800°C at 200–980 MPa, then rapidly cooled at 10°C / min, with pressure applied for 60 minutes), all showed no voids larger than 5.0 Å in diameter in positron annihilation lifetime analysis, and no ordered structure peaks larger than 8.0 Å in high-energy X-ray diffraction analysis (i.e., the intensity ratio to FSDP was 0.02 or less). In contrast, samples treated at 0.1 MPa (atmospheric pressure) (samples with a virtual temperature of 1069°C) showed numerous voids larger than 5.0 Å in diameter. In other words, the optical material of the present invention has structural fluctuations suppressed to a level that could not be achieved conventionally (substantially fluctuation-free).

[0036] Figure 6 shows the FTIR spectrum (absorption spectrum) (top figure) and the FTIR peaks separated by fitting, which are used to determine the virtual temperature (bottom figure). Figure 6 is included for reference purposes to show conventional knowledge (method for determining virtual temperature) and does not represent the measurement results of the optical material of the present invention.

[0037] Figure 7 is a graph showing the relationship between the Rayleigh scattering coefficient and the pressure of a silica glass sample subjected to high-temperature, high-pressure freezing treatment using a hot isostatic processing apparatus (treatment at 1800°C with 200-980 MPa followed by rapid cooling at 10°C / min for 60 minutes). The white circles "○" represent data previously calculated, while the data for the silica glass subjected to the high-temperature, high-pressure freezing treatment is shown as black squares "■". It can be seen that there is a minimum value at pressures below 1000 MPa.

[0038] Rayleigh scattering can be measured, for example, by placing a sample in a refractive index matching solution and irradiating it with laser light (488 nm), then measuring the scattering using a spectrometer and CCD at a position 90 degrees from the direction of the incident laser. By changing the type of sample, the Rayleigh scattering coefficient can be determined from the intensity ratio of the scattered light to that of a standard sample. Laser light is easier to measure because visible light scatters more. For fiber-shaped glass, one method involves determining the intensity of the light incident on the fiber and the intensity of the light measured at the exit end of the fiber, cutting the fiber to shorten its length, and determining the magnitude of the loss at the cut length from the change in the intensity of the exit light. In this method, since losses due to cutting are also included, multiple wavelengths of laser light are used for detection, and the component that follows the wavelength dependence (wavelength to the power of -4) of the Rayleigh scattering coefficient is extracted. Alternatively, a method (cavity ring-down method, CRD method) can be applied, which involves forming high-precision mirrors on both sides of the sample to be measured to create a resonator structure, thereby increasing the optical path length and amplifying the loss for measurement. The CRD method allows for the measurement of losses as the Q-factor of the resonator, and is expected to provide highly accurate loss measurements. Measuring the Q-factor of a resonator involves introducing pulsed laser light into the sample, or intermittently injecting light using a chopper, and measuring the time evolution of the light emitted from the sample. If the sample loss is large, the intensity of the emitted light rapidly weakens within the sample, and a short lifetime is observed. If the loss is small, the emitted light is observed to have a long lifetime, and the Q-factor can be calculated from this lifetime. Alternatively, the resonator lifetime can also be observed as the broadening of the spectral width of the emitted light (the lifetime and spectral width are related by the Fourier transform). However, since losses other than Rayleigh scattering may be included in this CRD method, it is preferable to perform a CRD method using multiple wavelengths to extract the Rayleigh component.

[0039] Spinning was performed by high-temperature melt-bonding a borosilicate glass cladding to a silica glass core. Because borosilicate glass has a high coefficient of thermal expansion at high temperatures, it undergoes significant contraction when the silica glass is in a supercooled liquid state, allowing compressive stress to be applied to the silica glass. Figure 8 shows how the Raman scattering spectrum of the core glass under pressure from the cladding during the fiber spinning process (core material after spinning, example) changes from the Raman scattering spectrum of the core glass in the preform state before fiber spinning (conventional silica glass without pressure treatment, comparative example 1). 800 cm⁻¹ －１ The Raman shift of the spectrum normalized by peak intensity is 600 cm⁻¹. －１ The intensity of the D2 peak, which indicates the number density of three-membered rings appearing in the vicinity, was compared. While it becomes clearer when the influence of the main peak's base is subtracted, the number density of three-membered rings decreased significantly due to the pressure applied during fiber spinning.

[0040] Figure 9 shows the total scattering spectrum of the core measured using 61 keV high-energy X-rays at the Spring-8 BL04B2 line, with the fibers lined up. The structure factor S(q) profile derived from the total scattering spectrum of the core glass under pressure from the cladding during the fiber spinning process (core material after spinning, example) is 0.8 Å. －１ No peak was observed at that point. The S(q) of the core glass in the preform state before fiber spinning (conventional silica glass without pressure treatment, Comparative Example 2) was 0.8 Å. －１ A clear peak was observed at that point.

[0041] Furthermore, the optical loss of the fiber after spinning according to the present invention was measured using the cutback method and was 0.09 dB / km at a wavelength of 1.55 μm. On the other hand, the optical loss of a standard fiber, measured using the same cutback method, was 0.15 dB / km (Comparative Example 3). Here, "standard fiber" refers to a general-purpose single-mode optical fiber (SSMF: Standard Single-Mode Fiber) conforming to the G.652 standard standardized by the International Telecommunication Union (ITU-T). The core uses silica glass doped with germanium to increase the refractive index, and the cladding uses pure silica glass. In contrast, the fiber preform (Comparative Example 2) was polished, a dielectric multilayer film was deposited on both ends, and the loss was measured using the cavity ring-down method. As a result, the Q value was measured with a 1.03 μm narrow-linewidth laser, and when converted to optical loss, it was 0.52 dB / km. This loss is considered to be due to Rayleigh scattering, so this value is used to determine the wavelength dependence of Rayleigh scattering λ. －４ Applying the rule to convert the value to 1.55 μm, it became 0.11 dB / km. In other words, the optical fiber of the present invention exhibited low optical loss comparable to bulk silica glass. Here, "bulk silica glass" refers to solid synthetic quartz glass before it is processed into specific shapes such as fibers or thin films. Because no heat is applied during the manufacturing process, it has characteristics such as low virtual temperature and defect density.

[0042] Furthermore, the nonlinear optical effects of the Example, Comparative Example 1, Comparative Example 2, and Comparative Example 3 were investigated. Since the Example and Comparative Example 3 were fiber-shaped, comparison using Z-scanning was not possible. Therefore, the nonlinear refractive index with respect to the incident laser light was measured using the cross-phase modulation (XPM) method. In the measurement, the pump light wavelength was 1.53 μm and the probe light wavelength was 1.55 μm. The nonlinear refractive index (n) of the standard fiber (Comparative Example 3) was... ２ ) is n ２ = 2.8 × 10 －２０ I understand ２ It was / W. In contrast, XPM was performed and the nonlinear refractive index n of the unknown fiber was determined from the phase shift amount of the probe light. ２ We estimated the nonlinear refractive index (n) of the optical fiber in the example.２ ) is n ２ = 2.0 × 10 －２０ I understand ２ / W. On the other hand, since Comparative Example 1 and Comparative Example 2 were bulk samples, Z-scan measurements were performed with a thickness of 1 mm. Comparative Example 2 was obtained by molding Comparative Example 1 and combining it with cladding to form a preform, so there was no difference in their nonlinear refractive indices, and both were n ２ = 3.0 × 10 －２０ I understand ２ The value was / W. In the above embodiment, a high pressure exceeding 100 MPa was applied from the cladding to the core during the fiber spinning process, and this pressure was substantially isotropic, and the material was cooled at a high cooling rate far exceeding 10°C / min. Therefore, analysis by positron annihilation lifetime measurement showed no voids with a diameter of 5.0 Å or more, and analysis by high-energy X-ray diffraction showed no ordered structure peaks of 8.0 Å or more (i.e., the intensity ratio with FSDP was 0.02 or less).

[0043] As described above, it was found that by subjecting silica glass to high-temperature, high-pressure freezing treatment under specific conditions, the minute voids present in the glass virtually disappear, and at the same time, the medium-range ordered structure of the atomic network constituting the glass also disappears. In other words, fluctuations in the network structure inside the glass are eliminated, which is thought to contribute to the reduction of Rayleigh scattering and the reduction of nonlinear optical effects. Furthermore, it was found that when fluctuations in the network structure inside the glass are eliminated, the increase in nonlinear refractive index is suppressed, while the refractive index improves despite the elimination of fluctuations, resulting in a unique optical property that could not be predicted from conventional knowledge.

[0044] Although we have described the present invention along with its embodiments, we do not intend to limit our invention in any detail of the description unless specifically designated, and we believe that it should be interpreted broadly without contradicting the spirit and scope of the invention as set forth in the appended claims.

[0045] This application claims priority based on Japanese Patent Application No. 2025-040744, filed in Japan on 13 March 2025, the contents of which are incorporated herein by reference as part of this specification.

Claims

1. An optical material made of silica glass that does not have voids with a diameter of 5.0 Å or more, as determined by analysis using the positron annihilation lifetime measurement method.

2. The optical material according to claim 1, wherein the silica glass does not have an ordered structure peak of 8.0 Å or greater in analysis by high-energy X-ray diffraction.

3. The optical material according to claim 1 or 2, wherein the silica glass comprises at least one atom of F, Cl, Al, P, Na, and K, and / or a hydroxyl group (OH).

4. The optical material according to claim 1 or 2, wherein the optical material is an optical material for suppressing Rayleigh scattering loss.

5. The optical material according to claim 1 or 2, wherein the optical material is an optical material for suppressing nonlinear optical effects.

6. The optical material according to claim 1 or 2, wherein the optical material is an optical material for suppressing an increase in nonlinear refractive index while increasing the refractive index.

7. The optical material according to claim 1 or 2, wherein the optical material is a material for forming an optical component.

8. The optical material according to claim 7, wherein the optical component is an optical communication fiber.

9. The optical material according to claim 7, wherein the optical component is an optical component used in a transmissive semiconductor exposure apparatus.

10. The optical material according to claim 9, wherein the optical component used in the transmissive semiconductor exposure apparatus is a lens, a photomask, a reticle, or a substrate.

11. The optical material according to claim 7, wherein the optical component is an optical fiber for a fiber laser.

12. Strength 1000W / cm ２ The optical material according to claim 1 or 2 for injecting the above-mentioned laser light.

13. The optical material according to claim 1 or 2, wherein the refractive index of the optical material at a wavelength of 1030 nm is 1.452 or higher.

14. A method for manufacturing an optical material, comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling it.

15. A method for modifying silica glass to suppress Rayleigh scattering of silica glass, comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling it.

16. A method for modifying silica glass to suppress nonlinear optical effects, comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling it.

17. A method for modifying silica glass to increase its refractive index while suppressing an increase in the nonlinear refractive index of the silica glass, comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling the silica glass.

18. A method for improving the refractive index of silica glass, comprising applying a pressure of 100 to 1500 MPa to silica glass heated and melted at 1600 to 2400°C, and then rapidly cooling it.

19. The method according to any one of claims 14 to 18, wherein the silica glass obtained by the above method does not have voids with a diameter of 5.0 Å or more as determined by analysis by positron annihilation lifetime measurement.

20. The method according to claim 19, wherein the pressure is applied by introducing an inert gas.

21. The method according to claim 20, wherein the heating and melting, the application of pressure, and the rapid cooling are performed using a hot isostatic processing apparatus.

22. The method according to claim 19, wherein the heated and molten silica glass is the core material of an optical fiber, and the pressure is applied to the heated and molten core material by causing the cladding material surrounding the core material to shrink in volume by cooling.

23. A method for manufacturing an optical component, comprising forming an optical component using an optical material or modified silica glass obtained by the method described in claim 19.

24. A method for manufacturing an optical fiber, comprising obtaining an optical material or modified silica glass obtained by the method of claim 22 as the core of an optical fiber.

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